LiPo battery/charger purchase sanity check

[Adam Smith]

Looking for someone with more knowledge about LiPo batteries than I do, just doing a sanity check before making an order. I'm planning on purchasing the following items, and want to ensure they're compatible:

https://hobbyking.com/en_us/multistar-high-capacity-4s-10000mah-multi-rotor-lipo-pack.html x8
https://hobbyking.com/en_us/accuell-s60-ac-charger-us-plug.html x4
https://hobbyking.com/en_us/turnigy-5a-8-26v-sbec-for-lipo.html x8

I intend to include these within a pair of Hal-Con costumes.
Each one would be using 2 batteries at a time, with 2 on charge or spares.
Chargers: How long would I expect a full charge to take? or would 2 chargers be enough, depending on charge time?
2 buck converters per battery, total capacity of 20A at 5V. Is this overkill? I don't suspect I'll be drawing much more than 8A at a time.

[Chris McDonald]

I would suggest going LiFe as the fire risk is much lower. I think I would feel safer with a bag of gasoline in my pocket than one of those 10Ah 4S LiPo's. A 4s 10000mAh would be quite the fire.

These batteries are a bit bigger but you will get the idea:

https://www.youtube.com/watch?v=CnNId0mDnBo

That charger has a max charge rate of 6A so it will take about An hour and 40 to charge a 10Ah pack from dead. Generally the batteries are designed to be charged at 1C (1 x the capacity) so they can be safely charged at 10A. No charge rate is completely safe with LiPo. Charge them outdoors away from anything that can burn. People have burned down there house charging LiPo's. Last summer one of the guys on our race team almost burned his 5th wheel down charging LiPo's for an RC truck. They caught it just in time to stop it from spreading. He had to replace the flooring and some curtains. 

LiPo's(and LiFe) really do not like to be over discharged. So you will want a low voltage alarm or better yet a low voltage cut off.

I would suggest going for a lower cell count if your running at 5V. You will get a little less conversion loss and you don't need high voltage sbec.

One thing nice about lithium batteries is that you can get the full capacity out of them. With lead acid you can only typically get like 50% of the rated capacity out of them. If you do go with the 4S 10,000 you should be able to run nearly 4 hours at 5V 8A draw. Depending on the switching regulator efficiency. 

  

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Chris

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[Chris McDonald]

Also note that those batteries are XT90 and the charger comes with XT60 connectors. 

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Chris

[Adam Smith]

The largest size LiFePo4 battery I see Hobby King offers is slightly less capacity that the LiPo one I linked above, and is over double the price.
https://hobbyking.com/en_us/zippy-flightmax-8400mah-4s2p-30c-lifepo4-pack.html
Is this the type you mean?

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[Chris McDonald]

Yea those are the batteries. Yes they are more expensive and don't typically come in as big of capacities.  But the risk of fire is much lower. 

Those batteries are rated for 30C. In your application you won't be exceeding 2C so you may be able to find cheaper batteries else where. 

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Chris

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[Adam Smith]

My original thought before looking at raw batteries was to use a high capacity USB power bank, possibly this one: https://www.amazon.ca/dp/B00M3073L4/
has 8x 18650's, outputs regulated 5v and charging stuff already built in. Would still need to wear 2 at a time to get 8A. Buy 8 total to get a set to charge while wearing. capacity is a bit lower, 134Wh. I'm estimating 350Wh to be able to last a full day.
Definitely safer, and way easier to implement however.

[Chris McDonald]

Those USB bank capacities are at 1S not at 5v or at 8S. So its 3.6V * 26800mAh = 96Wh  less conversion losses. Some of them run all the cells in parallel and then buck convert up to 5V.

That is still a pretty good deal if you can get by on the limited output. And you get built in charging, low voltage protection, over current protection, etc...

.

The downside will be the charge time. My guess would be 5-6 hours at 4A unless the charger is running at 8S voltage which would reduce it to an hour. 

With the RC packs your paying for the high current output. That LiFePO4 pack will do 30C so 30*8400mAh = 252A at 13.4V

[Adam Smith]

96Wh would still work, when using 2 sets of 2 for a total of 8 packs. having them charge overnight would be plenty of time.

[Adam Smith]

On Monday, 27 March 2017 11:30:49 UTC-3, Adam Smith wrote:

96Wh would still work, when using 2 sets of 2 for a total of 8 packs. having them charge overnight would be plenty of time.

I've looked through Amazon for other battery packs of similar size, I think this one has the best in/output currents: https://www.amazon.ca/dp/B01JIWQPMW
Dual inputs for 4A total, and 3 outputs at 3A each or max 6A total. Still at 96Wh